SHA-256 the file bytes, look for a recent (30-day) processed
data_import_batches row with the same hash, and short-circuit with
200 { duplicate: true, existing_batch_id, existing_po_id } when found.
Saves an AI extraction call + a cron tick for byte-identical re-uploads.
Stores file_hash on the new batch row so future re-uploads can match
against it.
Backed by a new index (idx_batches_file_hash on file_hash, status)
and a new file_hash column (text, 64 chars) on data_import_batches
applied via PB admin API.
Tests: 11 in this file (covering happy path, byte-identical re-uploads,
hash collisions, dedup-query failures, and existing edge cases).
d5b691e2ad